tracing: mark trace_test_buffer as __maybe_unused

After trace_selftest_startup_sched_switch is removed, trace_test_buffer()
is only used sometimes, leading to this warning:

kernel/trace/trace_selftest.c:62:12: error: 'trace_test_buffer' defined but not used [-Werror=unused-function]

There is no simple #ifdef condition that captures well whether the
function is in fact used or not, so marking it as __maybe_unused is
probably the best way to shut up the warning. The function will then
be silently dropped when there is no user.

Link: http://lkml.kernel.org/r/20171013142227.1273469-1-arnd@arndb.de

Fixes: d8c4dee ("tracing: Remove obsolete sched_switch tracer selftest")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_selftest.c

@@ -59,7 +59,7 @@ static int trace_test_buffer_cpu(struct trace_buffer *buf, int cpu)
  * Test the trace buffer to see if all the elements
  * are still sane.
  */
-static int trace_test_buffer(struct trace_buffer *buf, unsigned long *count)
+static int __maybe_unused trace_test_buffer(struct trace_buffer *buf, unsigned long *count)
 {
 	unsigned long flags, cnt = 0;
 	int cpu, ret = 0;